Update from Patrick's comments

- Only create threads when on main thread
- Make conditional on:
  - network.allow-experiments
  - network.dns.allow-srv-experiment
  - Telemetry::CanRecord
- Racily check mHasRunExperiment before doing string comparisons to save
  a bit of work in the common case
- Minor change to srv lookup name to make it more realistic
- Case-insensitive string comparisons
- Do experiment for first *.mozilla.org lookup we do, not just aus
- Comment for future WTF moments :)